Recently, standards have been set up and actually applied to various types of write-once or rewritable optical information storage media, including BD-R, BD-RE, DVD-RAM, DVD-R, DVD-RW and CD-RW. Meanwhile, technologies for performing read/write operations on those various optical information storage media compliant with the standards have also been developed and introduced into actual products.
A phase change type optical information storage medium is known as one of those various types of optical information storage media. If a phase change type optical information storage medium is irradiated with a laser beam, of which the intensity is higher than a predetermined value, a part of a recording film that has been irradiated with the laser beam changes its phases from an amorphous one into a crystalline one, thus forming a recording mark there. For example, a combination of spaces and marks, of which the lengths are determined by a signal that has been modulated to represent the information to write, is formed on tracks on the recording film. The amorphous and crystalline phases have mutually different reflectances. That is why if the tracks on which information is stored are scanned with a laser beam, of which the intensity is not so high as to cause a phase change on the recording film, reflected light, of which the intensity varies so as to represent the spaces and marks, can be obtained. As a result, the information stored on the optical information storage medium can be retrieved.
The recording film of a rewritable optical information storage medium may be made of GeSbTe as a recording film material, for example. Patent Document No. 1 teaches using a Te—O-M based material (where M is at least one element selected from the group consisting of metallic elements, metalloid elements and semiconductor elements) as a recording film material for a write-once optical information storage medium. The Te—O-M based material is a compound material, which includes Te, O and M and in which fine particles of Te, Te-M and M are randomly dispersed in a TeO2 matrix of the as-deposited material. When the recording film of such a material is irradiated with a laser beam, the portion of the recording film irradiated with the laser beam will melt to precipitate Te or Te-M crystals with large particle sizes. As a result, the reflectance of that portion that has been irradiated with the laser beam varies and the variation in the intensity of the reflected light can be detected as a signal. In this manner, a so-called “write-once operation”, which allows the user to perform a write operation only once, can get done.
Other than such a phase change type, also known is a method for forming a recording mark by stacking two thin films, made of mutually different inorganic materials, one upon the other, heating and melting together the two thin films with a laser beam, and blending and alloying them. Furthermore, also known is a method of making the recording film of an organic dye material. According to such a method, the organic dye is thermally decomposed with the heat generated by the laser beam radiated, thereby lowering the refractive index of the thermally decomposed portion of the recording film. As a result, compared to the other portions, which have not been irradiated with the laser beam and in which the organic dye has not been decomposed, the path length of the irradiated portion appears to have shortened with respect to the laser beam. Consequently, the irradiated and non-irradiated portions act just like the concave and convex pits of a read-only CD, for example, with respect to the incident light, and therefore, information can be read and written. In performing a mark edge recording operation on such a write-once optical information storage medium, the medium is irradiated with a laser beam consisting of multiple pulse trains (i.e., so-called “multiple pulses”), thereby causing a physical variation in portions of the recording film that have been irradiated with the laser beam and forming recording marks on the recording film of the storage layer. When a read operation is performed, on the other hand, information is retrieved as a variation in the intensity of the reflected light, which can be detected by a variation in reflectance.
Generally speaking, in a situation where marks and spaces to record need to be shortened to increase the storage density, if the length of a space that precedes a recording mark, among other things, is too short, then thermal interference will occur. That is to say, the heat generated at the rear end of mark recorded could conduct through the space portion and affect a rise in temperature at the frontend of the next mark. Or the heat generated at the frontend of the recorded mark could affect the cooling process at the rear end of the previous mark. Also, even if marks and spaces with accurate lengths have been formed on the tracks, the edge location of a short mark or space to be detected during a read operation could deviate from its ideal value according to the frequency characteristic of a reading optical system to be determined by the light beam spot size. This is a problem. Such a deviation of the detected edge location from the ideal value is generally called an “intersymbol interference”. The smaller the relative sizes of marks and space with respect to the light beam spot, the more significant the intersymbol interference. In that case, when a read operation is performed, the jitter and the bit error rate of the read signal will both increase. This is also a problem.
To minimize such intersymbol interference in conventional DVDs and BDs, the position of the first one of multiple pulses to be applied to form a mark is sometimes shifted according to the relation between the length of the mark in question and that of the space that precedes that mark. Or the position of the last one of multiple pulses to be applied to form a mark may be shifted according to the relation between the length of the mark in question and that of the space that follows that mark. Such a control of a write pulse position is usually called an “adaptive write pre-compensation”. By getting such adaptive write pre-compensation done, a write operation can be performed with the thermal interference of a recording mark compensated for in advance. Methods of making such adaptive write pre-compensation are disclosed in Patent Documents Nos. 2, 3 and 4.
Patent Document No. 2 discloses a rewritable optical information storage medium that stores in advance a write pulse standard condition. The write pulse standard condition specifies appropriate write pulse positions with respect to multiple possible combinations of various mark and space lengths. The storage medium also stores, in a predetermined area, method and location information for retrieving and modifying the standard write pulse condition and determining the best write pulse condition.
Patent Document No. 3 discloses a writing method in which respective marks to record are classified according to their own lengths and the lengths of their preceding and following spaces. According to Patent Document No. 3, the write pulse signal is controlled with the edge position of the second last one of multiple pulses in a write pulse train to record those marks shifted according to a result of the classification.
Recently, as the storage densities of optical information storage media have been increasing year by year, the lengths of recording marks are now getting closer and closer to the limit of optical resolution. As a result, the intersymbol interference is increasing and the signal to noise ratio (SNR) is decreasing more and more significantly. To cope with such a situation, someone proposed a method for determining a most likely signal sequence based on the waveform of a read signal obtained from an optical information storage medium by so-called PRML (partial response maximum likelihood) signal processing method, which is one of most likelihood decoding methods. For example, according to Non-Patent Document No. 1, if a PR (1, 2, 2, 1) ML method is adopted in combination with an optical system that uses a laser beam with a wavelength of 405 nm and an objective lens with an NA (numerical aperture) of 0.85 to read/write information from/on a BD with a diameter of 12 nm and a storage capacity of 25 GB (gigabytes) per side, a required system margin should be achieved. Also, that document also says that to write information on a BD with a storage capacity of 30 GB or 33.3 GB per side using the same optical system, the mark lengths should be decreased, the linear density should be increased, and the PR (1, 2, 2, 1) ML method should be adopted to process a read signal in that case.
Patent Document No. 4 discloses a writing control method for optimizing write parameters for use to write information by PRML, instead of the jitter of a read signal. According to such a method, a signal waveform is estimated by applying the PRML method to the waveform of a read signal, and the write parameters are optimized so as to minimize the probability of occurrence of errors by the signal waveform stayed.
Also, Patent Document No. 5 discloses that control information (such as write strategy type information) to get a read/write operation done properly on an optical information storage medium should be stored in an information unit in a predetermined area on the optical information storage medium. The document also says that different kinds of write strategy type information should be stored in advance on an information unit basis.